1. Field of Invention
The present invention relates to the preparation of metallocene compounds. More particularly, the present invention relates to a new process for the reaction of metal cations with cyclopentadienyl-type anions to form metallocenes.
2. Description of Prior Art
Metallocenes are being increasingly used as catalysts for the polymerization of olefins. Ewen and Razavi have recently disclosed that metallocene catalysts can be tailored to control the stereospecificity of polypropylene (JACS, 110, 6255-6, 1988). However, such control may be largely dependent on the purity of the catalysts.
Generally, the preparation of metallocene compounds consists of forming and isolating the cyclopentadienyl or substituted cyclopentadienyl ligand(s), which are reacted with a halogenated or alkylated transition metal compound to form a complex which is then purified.
European Patent Application No. 89870079.4 discloses two methods for the preparation of metallocenes.
In method A, n-butyllithium is added to a solution of the ligand in tetrahydrofuran (THF), after which metal chloride in THF is added with vigorous stirring. After refluxing, removal of the solvent leaves a mixture of LiCl and a red solid. However, this method gives metallocenes which are extremely-air and moisture-sensitive, and which are in addition somewhat impure so that they generally have to be purified by either pentane extraction, fractional recrystallization, or chromatography. The final complex nevertheless contains some THF, coordinated to the metal.
In method B, methylene chloride is used as a non-coordinating solvent. The reaction product of n-butyllithium with the ligand is isolated and dissolved in methylene chloride at the same temperature is added, and the mixture is allowed to warm slowly to room temperature before filtering off LiCl. However, this method gives a solution of an impure metallocene in methylene chloride, which must be recrystallized and washed. Further, this method requires the use of very low temperatures.
There is thus a need in the art for a method for preparing metallocene compounds that would not have the above drawbacks.